1. Field of the Invention
The present invention relates to a fixing belt used in an image forming apparatus such as an electrophotographic copier, a laser printer, facsimile or the like, a base element for the fixing belt and an image forming apparatus having the fixing belt. The present invention also relates to a method for producing the base element.
2. Description of the Related Art
FIG. 5 illustrates an explanatory view of a conventional image forming apparatus in an electrophotographic system. The conventional image forming apparatus 100 in the electrophotographic system, for example, a copying machine or a laser printer includes a photoconductor drum 101 on which an electrostatic latent image is formed, a charging roller 102 which is in contact with the photoconductor drum 101 to perform a charging process, an exposure device 103 such as a laser beam or the like, a development roller 104 which applies toner onto the electrostatic latent image formed on the photoconductor drum 101, a power pack 105 which applies a DC voltage to the charging roller 102, a transfer roller 106 which transfers a toner image formed on the photoconductor drum 101 onto a recording paper 107, a cleaning device 108 which cleans the photoconductor drum 101 after the transfer process, a surface potentiometer 109 which measures a surface potential of the photoconductor drum 101, and a thermal fixing device 110 which includes a thermal fixing roller 111 and a pressure roller 112.
In the image forming apparatus 100 using the electrophotographic system, a photoconductor layer of the rotating photoconductor drum 101 is uniformly charged by use of the charging roller 102, and then the photoconductor layer is exposed by the exposure device 103 such as a laser beam or the like to form the electrostatic latent image. The toner image is formed by developing the electrostatic latent image with the toner and then transferring it to the recording paper 107. The recording paper 107 is passed through the thermal fixing device 110 including the thermal fixing roller 111 and the pressure roller 112 so that the toner image is fixed by heat.
In the above-mentioned thermal fixing device of the image forming apparatus 100, for example, the thermal fixing roller 111 having a cored bar of a hollow cylindrical body made of aluminum or the like is provided with a separation layer of a fluorine resin or the like on an external surface of the cored bar to prevent the toner from being adhered thereon. A heater such as a halogen lamp or the like is disposed along a rotating center line in the hollow portion of the cored bar 80 that the thermal fixing roller 111 is heated from an inner side thereof by radiation heat of the heater. The recording paper 107 is passed between the thermal fixing roller 111 and the pressure roller 112 so that the toner attached on the recording paper is pressed and fixed thereon while being softened by the heat of the thermal fixing roller 111.
Although the thermal fixing roller 111 provided with the fluorine resin layer on the cored bar has good detachability, because of being less flexible and less elastic, the thermal fixing roller 111 cannot be applied to an image forming apparatus such as a full-color copying machine involving a glossed image, a full-color laser printer or the like. For the image forming apparatus such as a conventional full-color copying machine, the laser printer or the like, four color toners of red (magenta), blue (cyan), yellow and black are used and transferred on a paper surface in a superimposed state. Consequently, when the color toners in an unstable state are fixed, it is necessary that the toners be enveloped with a surface of the thermal fixing roller to allow the toners to be uniformly mixed in a melting condition. Therefore, flexibility is required for the surface property of the fixing member (roller, belt).
Conventionally, in order to provide the flexibility on the surface of the fixing roller, an elastic layer is provided on the fixing roller. The elastic layer such as a silicon rubber, a fluorine rubber or the like is provided on a silicon rubber roller, that is to say, a rubber material, that is to say, the elastic layer is made of a rubber material which has greater detachability and durability than that of a rubber material of the base rubber roller. However, in the fixing roller with the above-mentioned elastic layer, a thickness of the elastic layer is limited within a certain constant value to prevent a long warming up time. Since the thermal fixing roller allows a temperature thereof to be maintained at a desirable temperature and has a large heat capacity, it is suitable for speeding up fixing. However, since it takes a long time to heat the thermal fixing roller up to the desirable temperature, there is a problem in that a large power consumption is required.
Recently, in the field of image processing, methods of saving energy have been developed. As a result, in particular, the fixing device in a fixing belt type electrophotographic device has been used for fixing the color toner image with heat. That is to say, a fixing roller 5 is mounted on a fixing belt 1, and the fixing belt 1 is held and pressed by the fixing roller 5 and a pressure roller 6, as shown in FIG. 6. A transferred paper 8 is passed between the fixing belt 1 and pressure roller 6 to allow the toner on the transferred paper to be fixed as shown in FIG. 6. Moreover, a heating roller 2 is disposed on an end portion of the fixing belt 1, and a heater 3 such as a halogen lamp is disposed along a rotating center line in a hollow portion of the heating roller 2. The heating roller 2 is heated from an inner side by radiation heat of the heater 3 to transfer the heat to the fixing belt 1 via the heating roller 2.
Reference number 4 in FIG. 6 indicates a thermistor which measures a surface temperature of the heating roller 2 and turns on the halogen lamp 3 depending on the temperature. A tension roller 7 is disposed on the fixing belt 1 to prevent an occurrence of deflection when the fixing belt 1 is rotated.
FIG. 6B illustrates the fixing belt 1 including an endless belt 10 of a heat-resistant resin such as a polyimide or the like, an elastic layer 11 of a silicon rubber or the like, which is provided on an external surface of the endless belt 10, and a separation layer 12 of a fluorine resin, which is provided on a surface of the elastic layer 11 to prevent the toners from being adhered.
Since the fluorine resin layer as the separation layer is required to be provided on a surface of a base element, a polyimide resin as a heat-resistant resin which does not change at a melting temperature range of the fluorine resin, that is to say, 320 to 360° C. is preferably used for the base element.
A usual method for producing an endless belt from a polyimide resin includes applying a polyimide precursor solution on an inner surface of a cylindrical die made of a metal, drying the polyimide precursor solution by heating the cylindrical die from outside while rotating, then heating at 300° C. for several hours to harden and form a film, and removing the film from the die so that the endless belt is obtained.
In the above-mentioned fixing device in the fixing belt type electrophotographic device, the fixing belt is charged due to friction occurring when a transferred paper passes between the fixing belt and the pressure roller, so that a disadvantage such as an electrostatic off-set occurs. To prevent this charging due to the friction, Japanese Patent Laid-Open Publication Number Hei10-305500 discloses a technique of laminating a conductive silicon rubber which has a surface resistance of 105 to 1012Ω/□ on an external surface of a tubular fixing belt made of a conductive polyimide resin which has a surface resistance of 101 to 108Ω/□. Furthermore, Japanese Patent Laid-Open Publication Number Hei11-235765 discloses a method for producing a polyimide belt which is formed on an inner surface of the cylindrical die as mentioned above. The endless polyimide belt is formed from two layers including an insulation layer and a conductive layer which is disposed on an inner surface of the endless belt.
However, since a large number of dies are required and the inner surface of the die is required to be smoothed uniformly in order to allow the obtained endless belt to be separated from the die, these techniques involve high cost because of the dies.
Since the above-mentioned fixing device includes an expensive fixing belt, there is a need to decrease the production cost of the fixing belt.
As a method for producing a fixing belt at a comparably low cost, Japanese Patent Laid-Open Publication Number Hei05-185519 discloses a technique in which stepped portions are formed on opposite end portions of a heat-resistant film by a half-etching process with a UV laser. The opposite stepped portions are joined with a heat-resistant adhesive to form an endless belt. However, the technique does not achieve a charging prevention effect if the technique is applied to the fixing belt.